The provision of polymerbitumen strip for roof covering, utilizing the fact that adjoining strips of the roof covering can be thermally welded together, have generally made use of a carrier layer or insert, also referred to as a fiber reinforcement layer or blanket, which consists of a chemical fiber fleece, a glass fabric, glass fleece or the like which is covered by upper and lower sealing layers of polymerbitumen so that the carrier or the support can be found in the middle third of the sealing layer.
Such conventional weldable strips have the disadvantage that the upper and lower cover layers can easily pull away from the carrier so that the sealing layer itself is split. As a consequence the strips do not conform to the industrial standards for effective roof coverings and cannot assure the sealing of the roof surface to which they are applied.
Generally a fine gravel, e.g. of crushed slate, is applied as a protective layer to the upper cover layer of such roof coverings and, as a consequence, the penetration of sharp edges of gravel tends to destroy this upper cover layer.
Furthermore, when a cover layer of the aforedescribed type is provided above the reinforcement layer which is impregnated with the polymerbitumen, the gravel tends to spring away from the roof covering during the heating which results in the welding of the seam between adjoining strips of the roof covering causing a problem in the overlap or seam regions and presenting a danger to the workers applying the roof covering.
Furthermore, since gravel may be incorporated in the overlap region and the seam, a homogeneous bonding of two strips together in such overlap region is practically impossible.